Synthetic surfactant detergents, such as cationic, anionic, amphoteric, and non-ionic surfactants, are used widely in a variety of detergent and cleansing compositions to impart cleansing properties thereto. In addition, in certain compositions such as personal care compositions including shampoos and washes, it may be desirable to use combinations and levels of surfactants sufficient to achieve relatively high levels of foam volume and/or foam stability.
However, synthetic detergents tend to be irritating to the skin and eyes. As concentrations of such detergents in personal care compositions increase, so as to impart increased cleansing and foaming properties to these compositions, the irritation associated with such compositions also tends to increase, making such compositions undesirable for use on or near the skin and/or eyes.
Certain attempts to produce milder cleansing compositions have included combining relatively low amounts of anionic surfactants (which tend to be relatively high-foaming but also relatively highly irritating) with relatively lower irritating surfactants such as nonionic and/or amphoteric surfactants. See, e.g. U.S. Pat. No. 4,726,915. Another approach to producing mild cleansing compositions is to associate the anionic surfactants with amphoteric or cationic compounds in order to yield surfactant complexes. See, e.g., U.S. Pat. Nos. 4,443,362; 4,726,915; 4,186,113; and 4,110,263. Unfortunately, mild cleansing compositions produced via both of such methods tend to suffer from relatively poor foaming and cleansing performance.
Furthermore, in connection with certain uses, consumers desire cleansing compositions to be relatively clear. In particular, clear compositions are often used advantageously to provide an aesthetic indication of purity to the consumer. However, a number of ingredients commonly used in conventional personal care compositions, including, for example, polymeric thickeners, tend to cause the compositions to become cloudy or opaque. It is not readily predictable which combinations of polymers, surfactants and other optional ingredients may be combined to create compositions that are suitable for use as cleansers and also exhibit high clarity.
U.S. Pat. No. 6,897,253 ('253) describes a substantially crosslinked alkali-swellable acrylate copolymer rheology modifier, water, an alkaline material, and an effective amount of surfactant so that a substantially insoluble compound is stabilized or suspended. Such polymeric rheology modifiers require a pH of 5 or 6 in order to build substantial viscosity. The addition of a hydrophobically modifies polymer (“hmp”) to a surfactant system has been shown to result in a milder surfactant that still retains foaming performance (LiBrizzi et al., U.S. Pat. No. 7,157,414). Surface tensiometry has shown that the hmp associates a fraction of the surfactant to the hydrophobic domains of the polymer thereby reducing the free micelle concentration.
US 2008/0113895 describes the use of low molecular weight acrylic polymers with the anionic surfactants sodium laureth sulfate and sodium trideceth sulfate for mild cleansing systems.
U.S. Pat. No. 5,130,056 relates to a washing agent, cleansing agent and/or toiletry containing at least one ionic and/or amphoteric surfactant and at least one C8 to C18 fatty acid monoester of diglycerol and/or C8 to C18 fatty acid diester of tetraglycerol as a constituent of the mixture, 2 to 30% by weight, preferably 10 to 20% by weight, of at least one fatty acid monoester of diglycerol and/or fatty acid diester of tetraglycerol, relative to the total surfactant content (100% by weight), being present in the surfactant mixture.
However, these high molecular weight hydrophobically modified polymers lose significant efficiency at high polymer concentrations, that is to say as the hmp concentration is increased, the mildness benefit gets smaller and smaller.
US 2008/0112913 describes the use of low molecular weight acrylic polymers for irritation mitigation and points out the difficulty in creating clear cleansing systems with low molecular weight hydrophobically modified polymers.
More recently, low molecular weight hmp's have been shown to suffer less loss of efficiency compared with higher molecular weight hmp's (See U.S. Pat. No. 7,803,403). M. Fevola, r. Walters, J. LiBrizzi, “A New Approach to Formulating Mild Cleansers: Hydrophobically-Modified Polymers for Irritation Mitigation” Polymeric Delivery of Therapeutics, 2010, 221.) By reducing the molecular weight of the hmp, the polymer can more readily open into an expanded coil and therefore associate more surfactant even at higher polymer concentrations. Walters et al further demonstrated that this associated surfactant is in a more stable state, and the surfactant is less dynamic (co-pending U.S. patent application Ser. No. 12/779,211).
We have shown that low molecular weight hmp associates some fraction of the surfactant in the surfactant system, typically between 20-30%. The remainder of the surfactant is not associated to the polymer and exists as either free micelles or monomeric surfactant. With the low molecular weight hmp present, the surfactant exists in three states: 1) associated to the polymer 2) in a free micelle, or 3) as monomeric surfactant. The low molecular weight hmp only affects the surfactant that is associated to the polymer, so that the only mildness improvement to the formula is due to the surfactant associated to the polymer.
Even when the low molecular weight hmp is present, there are still many free micelles in solution that can contribute to the aggressiveness of the surfactant solution.
The skin care compositions of this invention have low irritation characteristics and are capable of exhibiting superior foaming, which is desirable in a cleansing composition.